1. Field of the Invention
This invention relates to a ceramic sheet and a process for producing the ceramic sheet, particularly, to a ceramic sheet which is resistant to cracking and breakage and has satisfactorily flat surfaces and highly stable qualities in the use as a material for planar solid oxide fuel cell (hereinafter referred to as “SOFC”) such as solid-electrolyte film, and so on.
2. Description of the Related Art
Planar SOFC have a basic configuration of a cell stack in which a number of cells are vertically stacked, each cell includes a solid-electrolyte film, an anode arranged on one side of the solid-electrolyte film, and a cathode arranged on the other side of the solid-electrolyte film. In such a cell stack, the individual cells are placed adjacent to one another, and separators (interconnectors) are interposed between the individual cells to avoid a fuel gas and air from mixing with each other. The periphery of the cell including the solid-electrolyte film is sealed and fixed with the separator. When the cell has a manifold inside thereof, at the periphery of the manifold, the cell is also sealed and fixed with the separator.
The separator is generally composed of a heat-resisting alloy or a ceramic having a high specific gravity, and is a thick sheet and therefore has a considerable weight. On the other hand, ceramic sheets for use as constitutive materials of the SOFC such a solid-electrolyte film should be lightweight and thin compared to the separator. The SOFC generally operates at high temperatures of about 800° C. to 1000° C., and the constitutive materials of the SOFC undergo significantly large thermal stresses in addition to heavy loads induced by stacking.
Zirconia sheets and other ceramic sheets are hard and are weak to external forces in a bending direction. If such ceramic sheets for use as solid-electrolyte films and other constitutive materials of SOFC have unevenness (protrusions and depressions), warp, waviness or other irregularities, the stacking-induced load and thermal stress concentrate at the irregularities to invite cracking and breakage to thereby rapidly deteriorate the performance of electric power generation.
The present inventors made intensive investigations to reduce cracking and breakage caused by the stacking-induced load and thermal stress in ceramic sheets for use as solid-electrolyte films of SOFC or the like, and to improve the performances and to prolong the life as SOFC. As part of these investigations, the present inventors found that the cracking and breakage can be minimized by controlling the warp and maximum waviness heights of sheets under specific levels, and proposed improved ceramic sheets (Japanese Unexamined Patent Application Publications No. 8-151270 and No. 8-151271).
The ceramic sheets disclosed in the above publications are capable of resisting the stacking-induced load and thermal stress even if they are significantly large-sized. The use of these ceramic sheets can largely increase the electrically generating capacity as SOFC and are expected to be a very effective technology for commercially practical use of SOFC.
However, the present inventors found the following facts during further investigations. Specifically, even in the a ceramic sheets having minimized warp and waviness disclosed in the above publications, cracking and breakage may occur according to the degree of the stacking-induced load or thermal stress. They also found that dimples on the surface of a ceramic sheet or burrs formed on the periphery of the sheet during punching a ceramic green sheet into a final shape largely affect the cracking and breakage.
When a ceramic sheet is used as the solid-electrolyte film or the like, about 50 to 100 plies or more of the ceramic sheets are stacked and assembled into a SOFC. Particularly in this case, when a large dimple or waviness exists on the surface of the sheet, a localized internal stress occurs in the dimple or waviness when plies of the ceramic sheets are stacked and assembled. If a stacking-induced load or thermal stress is applied during operation of the resulting SOFC, cracking and breakage are liable to occur in the region where the large dimple or waviness exists.
When a ceramic sheet is used as, for example, a solid-electrolyte film of a SOFC, the periphery of the sheet is sealed and fixed with a separator and is firmly restrained. If a large burr exists on the periphery of the sheet, a localized internal stress occurs at this point when the sheet is sealed and fixed, and cracking and breakage occur at the point when a stacking-induced load or stress is exerted on the sheet during operation of the SOFC.
Such a ceramic sheet is generally prepared by a process including the steps of preparing a slurry containing a ceramic material powder, an organic binder, and a dispersion medium, molding the slurry into a sheet by, for example, the doctor blade process, calendering process, or extrusion process, drying the molded sheet to remove the dispersion medium through volatilization to thereby yield a green sheet, punching the green sheet into a predetermined shape, and baking the punched green sheet to decompose and remove the organic binder and to sintered body the powdered ceramic with each other. In the sintering step, the green sheet shrinks to about 70% to 90% in length and to about 50% to 80% in area. Unevenness of the decomposition and emission speed of the organic binder within the green sheet surface during the sintering step or non-uniform of the shrinkage accompanied with sintering in the green sheet surface will cause a large dimple on the surface of the resulting ceramic sheet. If a burr forms on the periphery of the green sheet in the step of punching the green sheet into a predetermined shape, the burr is supposed to substantially remain in the resulting ceramic sheet and may invite defects of product.
Zirconia ceramic sheets are known as representative examples of ceramic sheets having satisfactory performances as solid-electrolyte films of SOFC. Among them, a zirconia ceramic partially stabilized with 2.8 to 4.5 mole percent of yttria is a ceramic obtained by stabilizing tetragonal crystals at room temperature, which tetragonal crystals are originally stable at high temperatures. This partially stabilized zirconia ceramic is supposed to have very high mechanical strength and toughness at room temperature when compared with a fully stabilized cubic zirconia containing 6 to 12% by mole of yttria. The reasons are supposed as follows.
When such a partially stabilized tetragonal zirconia ceramic is subjected to a stress and a fine crack locally occurs, the phase of a tetragonal crystal in the stress field at the edge of the crack is transferred from tetragonal to monoclinic by action of the stress, and a volume expansion accompanied with the phase transition absorbs the stress to thereby inhibit the progress of cracking. However, at high temperatures exceeding 800° C., the tetragonal crystal is stabilized and the ease of the phase transition from tetragonal to monoclinic is decreased, and the toughness of the ceramic is supposed to markedly decrease to a level similar to that of a fully stabilized cubic zirconia ceramic. When such a zirconia ceramic is used as a molded sheet, the surface on which the stress is applied become significantly large relative to the total volume of the sheet, and the aforementioned phenomena markedly occur.
For example, the bending characteristics of high strength zirconia sintered body are described on page 38 of “New Material Manual 1987 (4th Edition)” (published on Mar. 27, 1987 from Techno Plaza Co., Ltd., Japan), in which the bending strength of the zirconia sintered body is 120 to 170 kgf/mm2 (about 1.2 to 1.7 GPa) at room temperature but is decreased to 30 to 40 kgf/mm2 (about 0.3 to 0.4 GPa) at 1000° C.
On the other hand, ceramics are satisfactory in heat resistance and abrasion resistance and other mechanical characteristics and also in electrical and magnetic characteristics and are therefore employed in a variety of applications. Of these ceramics, ceramic sheets essentially composed of zirconia have a satisfactory oxygen ion conductivity, heat and corrosion resistance, toughness, chemical resistance and are effectively used as solid-electrolyte film(s) for a sensor member such an oxygen sensor or a humidity sensor, as well as solid-electrolyte film(s) for SOFC as stated above.
Attempts have been made in which alumina and other additional components are incorporated into the zirconia ceramic. For example, Japanese Unexamined Patent Application Publication No. 2-177265 proposes a technique of adding 5 to 20% by mass(weight) of alumina to a partially stabilized zirconia and sintering. The zirconia containing alumina in order to avoid the formation of monoclinic crystals even in a long-term use at high temperatures, to inhibit the decrease of conductivity even at high temperatures and to obtain a solid-electrolyte film having satisfactory mechanical strength. This publication states that the addition of alumina in the above-specified proportion to the partially stabilized zirconia inhibits the phase transition from tetragonal to monoclinic even at high temperatures to thereby increase the mechanical strength and stability of the ceramic.
However, even though the mechanical strength of the solid-electrolyte alone can be improved by the addition of alumina, when the solid-electrolyte is assembled with electrodes into a cell, the alumina and electrode components are supposed to undergo a solid-phase reaction at high temperatures to form an alumina complex oxide having a low conductivity. The resulting cell is therefore supposed to be low in mechanical strength.
Japanese Unexamined Patent Application Publication No. 6-64969 discloses a zirconia ceramic containing a decreased amount of alumina. This ceramic is a zirconia solid-electrolyte film containing 2 to 12% by mole of yttria as a solid solution and 0.01 to 2% by mass of alumina. This ceramic is mainly directed to suppress changes with time of electrical conductivity when an electric current is continuously passed but is not directed to improve the strength.
U.S. Pat. No. 4,886,768 discloses a toughened tetragonal zirconia prepared by adding 0.5 to 3% by mole of Ta2O5 or Nb2O5 as a toughening agent to a zirconia stabilized with 2 to 4% by mole of Y2O3. This zirconia does not have sufficiently highly stable mechanical strength over time. This is probably because Ta2O5 or Nb2O5 is added in an excessively large amount.
Attempts to improve strength at high temperatures of partially stabilized zirconia ceramics are still not sufficiently made.